An electric power steering device is used as a unit that generates steering assist torque and configured so as to generate the torque by detecting the steering force of a driver with a torque sensor and controlling the magnitude and direction of a current, which is supplied to a motor, by a bridge circuit in response to the steering force, and a common configuration thereof includes, as principal constituent elements, a motor for assist, a power supply such as a battery, and a bridge circuit that supplies a current amount to the motor in response to the assist torque between the motor and the power supply.
In such a configuration, various techniques are developed and proposed. For example, WO 2005/081386 (PTL 1) deals with, as a problem, a situation in which, when the voltage of a power supply is boosted to a predetermined voltage value and then supplied to a motor, if a mechanical relay contact is used, discharge occurs between the relay contacts at the time of switching from ON to OFF and causes the relay contacts to be welded, which greatly affects the circuit operation, and proposes, as measures for preventing such a situation, monitoring the potential on the input side of the relay contact and performing control to turn the relay contact OFF if the value of a voltage difference between the input side and the output side of the relay contact drops below a predetermined threshold value. Since this proposition is based on the assumption that a mechanical relay contact is used as a relay contact, it may be possible to switch this relay contact from a mechanical relay contact to a semiconductor switching element. Such a configuration uses an N-channel semiconductor switching element as a relay as proposed in Japanese Patent No. 3375502 (PTL 2), for example, and turns ON/OFF a current which is supplied from a power supply by controlling the current flowing between the source and the drain by a gate voltage.